List out of range while comparing

Question

I'm trying to see if sum of two objects in a list is equal to a number or not. I wrote this code and it gave me an error - list out of range, when I try printing the index of the objects. I don't see the error, when I print the objects -

ls = [2,8,12,7]
for x in ls:
    for y in ls[1::]:
        if x + y == 9:
            print(ls[x],ls[y]) # gives error
            print(x,y) # works fine

I was able to resolve it by using enumerate function but would like to know why I'm getting an error for the above code.

Working code -

ls = [2,8,12,7]
for inx,x in enumerate(ls):
    for iny,y in enumerate(ls[1::]):
        if x + y == 9:
            print(inx,iny)

Answer 1

When you iterate over a list with a for loop, e.g. for x in ls, you're iterating over the values of the list, not the indexes. Therefore, x will be 2, 8, 12, 7, in that order. And since your list only has 4 elements, there is no element at index 8, which is causing the index out of range exception when trying to access ls[x].